One of the defect factors of the LCD device is either a non-lighting state of the light source or a brightness decline due to lifetime of the light source. Since these defects can be restored easily by replacing the light source, it is important to have an interchangeable mechanism or a detachable mechanism for a light source in the LCD device for a long-term use.
The above-mentioned detachable light source is described with reference to FIG. 27, which corresponds to FIG. 2 of the Japanese Patent Application Laid-Open No. 1994-051293 (patent document 1).
A detachable light source unit 271 disclosed in the patent document 1 is provided with light source holding parts 276 at both ends thereof and a reflective body 279 is extended between the holding parts to accommodate a light source 278 therein. This light source unit 271 is designed to sandwich a light guide plate (LGP) 275 with a light guide plate holding parts 270 and being held between a holding case 272 and slide system 277. The light source unit 271 is able to slide in parallel to the LGP 275 so that the light source unit 271 is enabled to be pulled out or ejected as a whole from the holding case 272 and thereby providing the detachable light source unit.
A straight pipe type cold cathode fluorescent tube is used as the above-mentioned light source which is operated with exothermic temperature higher than that of the LED (Light Emitting Diode). The straight type cold cathode fluorescent tube is a stick-like light source, and it emits light except for end portions adjacent to electrodes at both ends. When such the fluorescent tube is used as the light source, the reflective body 279 covers the fluorescent tube 278 except for the neighborhood of both ends around the light source. Only a part of the reflective body 279 opposing to an optical incident plane of the LGP 275 is opened to direct the light of the light source enter into the LGP 275. In order to avoid influence of heat from the light source, the reflective body 279 and the LGP 275 are generally separated from the light source. Accordingly, a distance between the light emitting surface of the light source and the optical incident plane of the LGP slightly affects its optical characteristic.
On the other hand, when an array of the LEDs is used as the light source, each of the LEDs is a point-like light source and thus requiring efficient usage of the output light. Therefore, the distance between the light emitting surface of the LED and the optical incident plane of the LGP greatly affects its optical characteristic. In order to take full advantage of the light of the point light source as the backlight module of the LCD device, it is necessary to enter the output light into the LGP without leaking the output light from the point light source. To this end, it is required to place the light emitting surface of the LED as close as possible to the optical incident plane of the LGP, and as shown in FIG. 28, which corresponds to FIG. 1 of the Japanese Patent Application Laid-Open No. 2005-321586 (patent document 2), the structure that makes an LED light source 28 touch onto an LGP 25 is general and effective. In FIG. 28, the LED light source 28 is mounted on a flexible printed circuits (FPC) substrate 29 and the FPC substrate 29 is fixed to the LGP by using a double-stick tape 259. The LGP 25 is sandwiched between a reflective sheet 26 and a group of optical sheets 24 and they are housed within a case 27 and covered by a pushing frame 22.
Since the array of LEDs 28 is the array of point-lights opposing to the optical incident plane 255 of the LGP 25, an array of bright spots due to the LEDs tends to occur at the light emitting surface of a backlight module or the liquid crystal display surface. In order to avoid such occurrence, special treatments are applied to the optical incident plane 255 of the LGP 25 or opposing surfaces of the LEDs 28 and the LGP 25 so as to eliminate dot-like bright spots due to LED emission by dispersing the output light of the LEDs 28, and thereby equalizing the brightness of the light emitting surface 251 or the LCD surface of the backlight module as shown in Japanese Patent Application Laid-Open No. 2006-302710 (patent document 3) and Japanese Patent Application Laid-Open No. 2004-213025 (patent document 4). FIG. 29 and FIG. 30 correspond to FIG. 3 and FIG. 1 of the patent document 3, respectively, and FIG. 31 corresponds to FIG. 6 of the patent document 4.
In FIG. 29 and FIG. 30, the array of the LEDs 28 is provided on the wiring substrate 29. A prismatic group 256 is formed on the optical incident plane of the LGP 25.
In FIG. 31, a refraction member 285 is interposed between the LEDs 28 and the LGP 25. The refraction member 285 is provided with light transmission areas 284 on the side surface thereof opposing to the optical incident plane 255 of the LGP 25 through which a central optical axis 289 of each of the LEDs 28 passes, and prismatic areas 286 are formed on both sides of each light transmission area 284. The hologram pattern 256 is provided on a rear side of the LGP 25.
When the cold cathode fluorescent tube is used as the light source, in view of considering influence of the heat of the light source and such structure that the light source is stick-type and the whole light source emits light while the light source is covered with the reflective body, it is effective to separate the light source and the LGP, and such light source interchangeable mechanism as shown in patent document 1 can be used.
However, when a point light source like the LED is used as a light source (hereinafter, referred to as LED), the light emitting surface of the LED needs to be brought as close as possible to the optical incident plane of the LGP as mentioned above. Accordingly, when a mechanism of a detachable light source shown in the patent document 1 is applied to the LED light source unit, the light source and the LGP are touched each other at a time of inserting or ejecting the light source unit, and causing such a problem that the light emitting surface of the LED, the optical incident plane of the LGP or a specially treated portion would be damaged and result in a display defect such as bright spots or dark spots.
When the gap of the LGP and the light source is made large to avoid the above mentioned problems, because the distance of the optical incident plane of the LGP and the light emitting surface of the LED greatly affects the optical characteristic, a problem is caused such that a utilization efficiency of the emitted light of the LED decreases remarkably.